THE SYSTOLIC OUTPUT OF THE HEART. 53 



confined in the pericardium, and these drugs be pushed to excess, the 

 heart ceases to have room for diastole when the distension reaches a 

 certain point ; cutting the pericardium relieves the heart for a time, but 

 final cessation in diastole takes place eventually. The circulation of 

 calves' blood through dogs' hearts has the same effect. 1 The diastolic 

 distension of the heart can be immediately relieved by placing the body 

 in the vertical feet-down position ; the right heart is thus emptied into 

 the splanchnic area. 2 



The action of the inhibitory and accelerator nerves on the output 

 of the heart is of considerable interest. 



The effect of excitation of the peripheral end of the vagus nerve 

 has been recorded by the cardiometer. 3 The output per second is 

 lessened, the residual blood is increased, the expansion of the heart in 

 diastole is made more evident. The excitation of this nerve weakens or 

 arrests the auricular contractions, and, while it slows the ventricles, the 

 venous pressure rises because the output is lessened. A considerable 

 rise of pressure in the left auricle accompanies the great fall of aortic 

 pressure. 4 



By moderate stimulation of the vagus, the output per second may be 

 diminished 20 to 30 per cent. The ventricles are more expanded both in 

 diastole and in systole, owing to the increase of the diastolic filling pressure. 

 Excitation of the accelerator nerves, on the other hand, increases the out- 

 put per second by augmenting the force of the heart, and especially that 

 of the auricular contraction. The venous pressure falls in consequence, 

 the arterial pressure rises, and less residual blood remains behind within 

 the ventricle. As a rule, the output per systole is, owing to the increase 

 of diastolic filling, greater when the heart beats slowly, less when the 

 heart is accelerated. The opposite is true for the output per second. 



So long as the output per second remains unchanged, the arterial 

 pressure will not vary, other factors remaining constant ; but if the 

 output decreases the pressure will fall, and if the output increases the 

 pressure will rise. Changes in cardiac frequency, however, affect the 

 mean arterial pressure only when the peripheral resistance is sufficiently 

 great, but not otherwise. 



Thus, if the arterioles are in a state of tone and the peripheral 

 resistance is considerable, then acceleration of the heart raises the mean 

 arterial pressure. Such, for example, is markedly the case when both 

 vagi are divided in a morphinised dog. The blood is then pumped 

 into the arteries faster than it can escape through the arterioles. On 

 the other hand, when the peripheral resistance is greatly diminished 

 by section of the spinal cord in the cervical region, then, on dividing 

 both vagi, or exciting the accelerator nerves, only an insignificant 

 change of mean arterial pressure is to be observed. 5 In this case the 

 blood escapes with great rapidity into the veins. This is a fact of the 

 utmost importance, for it is evident that unless the arterioles are closed 

 down to a certain extent, the heart cannot, between its contractions, 

 maintain the arteries in a condition of distension. 



1 Newell Martin, Phil. Trans., London, 1883, vol. clxxiv. p. 663. 



2 Hill, Brit. Med. Joum., London, 1897. 



3 Roy and Adami, Phil. Trans., London, 1892, vol. clxxxiii. B, p. 215. 



4 Kauders, Zlschr. f. klin. Med., Berlin, 1892, Bd. xxi. S. 67. 



5 Miinzel, Arch. f. Physiol., Leipzig, 1887, S. 136; Francois-Franck, "Tiav. du 

 labor, de Marey," 1880, Bd, IV. S. 296. 



